1. Field of the Invention
The present invention relates to a flow rate control device for controlling a flow rate of a liquid, particularly, to a flow rate control device for controlling a flow rate ranging from a small rate to a large rate.
2. Description of the Related Art
Generally, a flow rate of a liquid is determined by a valve opening (a cross section of an opening) of a valve and by a pressure differential across the valve. There are two types of device for controlling the flow rate of the liquid. One changes the valve opening, and another changes the pressure.
A curved line 100 shown in FIG. 9 represents a change of the flow rate when the valve opening is changed in the state that the pressure is relatively high. A curved line 102 represents a change of the flow rate when the valve opening is changed in the state that the pressure is relatively low. A curved line 104 represents a change of the flow rate when the pressure is changed in the state that the valve opening is relatively great. A curved line 106 represents a change of the flow rate when the pressure is changed in the state that the valve opening is relatively small. When the pressure is changed to change the flow rate, the flow rate is proportional to the square root of the pressure.
By changing the valve opening, a relatively wide-ranging flow rate can be controlled, as shown in the curved lines 100 and 102. However, in this case, the change of the flow rate reflects directly the change of the valve opening. In the extremely-low flow rate range, the flow rate is greatly changed in response to a small change of the valve opening, and thus not controlled accurately. Additionally, disturbance of the pressure change also influences the change of the flow rate.
When the pressure is changed, the valve opening is set small to stabilize the flow rate. For example, the flow rate is controlled along the curved line 106. In this case, because the flow rate changes small in response to the change of the pressure, stability of the flow rate is increased. On the other hand, because a range of the controllable flow rate is narrow, a high flow rate cannot be controlled.
A device for stably controlling a flow rate in response to a temperature change (for example, see a patent document 1), a device for reducing influence of a flow rate change caused by a change of upstream pressure (for example, see a patent document 2), and a device for controlling flow rates of more than two liquids and for mixing the liquids (for example, see a patent document 3) are suggested as the flow rate control device for controlling a flow rate of a liquid.    [Patent Document 1]    JP-A No.124460/1995    [Patent Document 2]    JP-A No. 303609/1997    [Patent Document 3]    JP-A No. 161751/1998
In principle, the devices suggested in the patent documents 1 to 3 control a flow rate by changing a valve opening, and thus the flow rate is instable due to disturbance such as a pressure change. Additionally, to control an extremely-low flow rate, the valve opening needs to become extremely small, and to be changed by an extremely-small degree. Therefore, the mechanical structure becomes complicated, and an accuracy, high cost motor is required.
Recently, a chemical liquid used in, e.g., semiconductor equipment is required to be supplied appropriately at an extremely-low flow rate to save an amount of the liquid. In this case, the flow rate is required to be accurately controlled in the state that the flow rate is extremely low. On the other hand, because a flow rate of pure water used for dilution and mixture of the chemical liquid is high, it is preferable that such a high flow rate is controllable.
In mixing the chemical liquid and pure water, a difference between pressures of the chemical liquid and pure water could cause the flow rate to be instable. In other words, when a pressure upstream of the flow of the chemical liquid is set low to make the flow rate low, a back pressure could cause the flow rate to be instable in mixing the chemical liquid and the pure water having a high flow rate.
Further, e.g., pressure pulsation by a pump supplying the chemical liquid is a factor of disturbance in the flow rate control.